Alkali beam cells can be utilized in various systems which require extremely accurate and stable frequencies, such as alkali beam atomic clocks. As an example, alkali beam atomic clocks can be used in bistatic radar systems, global positioning systems (GPS), and other navigation and positioning systems, such as satellite systems. Atomic clocks are also used in communications systems, such as cellular phone systems.
An alkali beam cell typically contains an alkali metal. For example, the metal can be Cesium (Cs). Light from an optical source can pump the atoms of an evaporated alkali metal from a ground state to a higher state, from which they can fall to a different hyperfine state. An interrogation signal, such as a microwave signal or intensity modulated light beam, can then be applied to the alkali beam cell and an oscillator controlling the interrogation signal can be tuned to a particular frequency so as to maximize the repopulation rate of the initial ground state. A controlled amount of the light can be propagated through the alkali beam cell and can be detected, such as by a photodetector, to form a state detection device.
By examining the output of the detection device, a control system can provide various control signals to the oscillator and light source to ensure that the wavelength of the propagated light and microwave frequency are precisely controlled, such that the microwave input frequency and hyperfine transition frequency are substantially the same. The oscillator thereafter can provide a highly accurate and stable frequency output signal for use as a frequency standard or atomic clock. However, Doppler broadening of the measured hyperfine transition frequency can occur as a result of non-orthogonal planar movement of the evaporated alkali metal atoms relative to the optical source, such as resulting from the random thermal motion of the alkali metal.